Instructions on how to rotate the tower in Myst library and get the access key to unlock the tall tree.
Rotate the Tower
In the library on Myst, look at the painting of Myst Island, and click and hold the blinking circle until the line turns red. Make the line point to the hexagon, which corresponds to the tall tree next to the cabin.
Click the painting of the stairs in the library to make the door to the elevator
open. Go up and find the access key for the tall tree:
Enter the Cabin
Go into the cabin on Myst island, then turn around to face the door. Click on the safe, and put in the combination 7,2,4. Pull the lever to open the safe. You find a box of matches. Open it and click it to take out a match, then move the cursor over the brown stripe on the side of the box of matches to light the match. Turn around and put the mouse over the right side of the red wheel, then click to start the flow of gas to the furnace. Move the cursor over the square on the bottom left of the furnace to light the flame.
Raise the Tree
Keep rotating the red wheel until the red needle in the gauge on the furnace starts to go up. You will hear a noise outside. After you hear this noise several times, if you go outside and look at the tree, you will find that it has gone up. Depending on how far the tree has gone, you will see a door in it. If the tree has gone too far up, or not far enough, you won't be able to enter the door, so you will need to rotate the wheel in the cabin to change the pressure.
You need to make the tree go up until you can enter the door, then turn the wheel to make the tree start to go downward, so that when you go into the door, you will go downward with the tree. If you go into the tree while it is going up, look for a button inside of the tree. You can hold this button to make it go back to ground level. This button can't make it go farther down, though, so you will need to go back to the cabin and use the wheel to make it start going downward.
Once the tree elevator takes you down to the room underground, go forward and use the linking book to go to Channelwood Age. If you want to go back up instead, you can turn around to face the elevator, and rotate the wheel there to make it start to go upward again.